1. Field of the Invention
The present invention relates to a mass spectrometer and method of controlling it.
2. Description of Related Art
A quadrupole mass spectrometer is an instrument which has a quadrupole mass filter generating a hyperbolic electric field, produces a selecting voltage by superimposing an RF voltage and a DC voltage on each other, and passes ions of only a desired mass-to-charge ratio by applying the selecting voltage and an axial voltage (that is a DC offset voltage applied to the four quadrupole electrodes equally) to the mass filter. A mass spectrum of the sample is obtained if the mass-to-charge ratio of selected ions is varied in equal increments. This method of measurement for obtaining a mass spectrum is known as scanning. In scanning, the RF voltage and DC voltage applied to the quadrupole mass filter are swept finely.
Sometimes, ion cooling is done on the upstream side of the quadrupole mass filter. In the cooling, ions are normally caused to collide with a gas by a multipole ion guide. The collision with the gas lowers the average kinetic energy of the ions and also reduces the range of kinetic energies. The cooling makes uniform the velocities of ions which are about to enter the quadrupole mass filter. This leads to improvements of resolution and sensitivity.
If two quadrupole mass filters are coupled together and a collisional cell is mounted between them, a triple quadrupole mass spectrometer is built. Since a triple quadrupole mass spectrometer has the two mass analyzers, it provides higher ion selectivity than a single quadrupole mass spectrometer and is often used in quantitative and qualitative analysis.
In a triple quadrupole mass spectrometer, desired ions are first selected by the first mass analyzer. The ions selected by the first mass analyzer are normally known as precursor ions and guided into a collisional cell including a multipole ion guide. An entrance electrode and an exit electrode are disposed at the opposite ends of the ion guide. The ion guide has means for introducing a gas from the outside via a needle valve. If a gas is introduced into the collisional cell, precursor ions collide against the collision gas, producing fragmentation with a certain probability. As a result, the precursor ions are fragmented in the collisional cell. These fragmented ions are known as product ions. Only intended ions of the precursor ions and the product ions in the collisional cell are separated by the second mass analyzer and detected. In a triple quadrupole mass spectrometer, product ions are normally measured and, therefore, the collisional cell is required to have high fragmentation efficiency.
Storage and ejection of ions allow for miniaturization of the instrument. In a quadrupole mass spectrometer or a triple quadrupole mass spectrometer, it is difficult to shorten the quadrupole mass filter because the resolution will be deteriorated by such shortening. To achieve a reduction in instrumental size, it is urged to shorten the multipole ion guide and/or the collisional cell. If these portions are shortened, the number of collisions with the collision gas decreases normally. This will hinder ion cooling or fragmentation. If a large amount of collision gas is introduced to maintain a sufficiently large number of collisions, the pressure in the latter stage of mass analyzer will increase. This may lead to a decrease in sensitivity. However, if a gas is stored temporarily, the ions repeatedly collide with the collision gas while reciprocating between the entrance and exit of the multipole ion guide or collisional cell. Therefore, if the amount of introduced gas is suppressed, a number of collisions necessary for cooling and fragmentation can be secured. As a result, the size of the instrument can be reduced.
In the case of high-speed scanning where the selected ion is varied while one ion is passing through the quadrupole mass filter, it is generally desired to maintain constant the amount of ions entering the quadrupole mass filter in a given time. On the other hand, where ions are stored and ejected, ejection is done intermittently. Therefore, ions entering the quadrupole mass filter assume the form of pulsed ions. If high-speed scanning is done in a quadrupole mass filter into which pulsed ions are passed in this way, there is the possibility that a mass spectrum inaccurately reflecting temporal information about pulsed ions might be observed. For example, no ions enter during the period between two successive pulsed ions. Ions of the mass-to-charge ratio selected during this period have zero intensity. In order to observe a mass spectrum representing intrinsic properties of the sample, the ion selected by the quadrupole mass filter must not be varied while pulsed ions are passing through. As a result, in a triple quadrupole mass spectrometer where ions are stored and ejected, it is difficult to achieve high-speed scanning.
On the other hand, in almost all cases of quadrupole mass spectrometers and triple quadrupole mass spectrometers, a chromatograph is used as a pretreatment unit. In recent years, chromatographs operated at amazingly increased speeds have become available. With this trend, there is an increasing demand for higher-speed scanning of mass spectrometers.